By Lani van Vuuren

Lani van Vuuren

THE pollution of South Africa's surface water has received much attention over the years, but degradation of underground supplies has gone largely unnoticed - until now.

The Water Research Commission (WRC) of South Africa has of late funded intense research on the extent of contamination of the country's aquifers by dense, non-aqueous phase liquids (DNAPLs).

DNAPLs are chemicals or mixtures of chemicals heavier than water, and which are only slightly water-soluble. This means that when released into the environment in sufficient quantities, they can move through soils and groundwater until they encounter an impermeable layer that causes the liquid to pool, and move along the layer's slope or within a fracture.

These DNAPLs can thus move in a direction opposite to that of the localized groundwater flow, meaning contamination may spread in any direction.

Contamination with DNAPLs is a worldwide phenomenon, but until now there has been limited awareness and understanding of the hazard in South Africa.

There are thousands of potential sites in the country, as elsewhere in the world, where DNAPLs may have been released to the subsurface. Examples include chlorinated solvents (degreasers and dry-cleaning fluids), creosote, coal tar, and polychlorinated biphenyls (as found in dielectric oils, for example).

Public awareness of DNAPL pollution has been very low. WRC research manager Dr Shafick Adams says South Africans must be acquainted with the hazards DNAPLs pose to the environment, and specifically to groundwater.

"Because of the low solubility of DNAPLs they often persist for several decades in the subsurface. In addition, some DNAPLs are highly toxic at even very low concentrations, and pose unacceptable health risks to humans and the environment."

DNAPLs are not often released into the environment as pure chemicals, but rather as solvents containing varying amounts of other contaminants. These other components can significantly influence the overall properties of the DNAPL, and aid detection as well as complicate remediation.

DNAPLs trapped below ground act as sources of dissolved contaminant to groundwater. Their removal from aquifers is very difficult and costly. As a result, these chemicals have been classified as high-priority pollutants in many countries, including South Africa.

While major industries are increasingly aware of the hazards of organic pollution, there is often no routine monitoring for organic compounds, and this has become an urgent need. When the low analysing capacity in South Africa is added to the equation, one can begin to understand why DNAPL contamination often goes undetected.

"Large operators sometimes have systems to monitor organic pollution stemming from leaks and accidental spillage," says Dr Adams. "However, until recently there were no specific guidelines for the assessment of DNAPLs in the country."

The first thorough, publicly-funded study into the nature and transporting of DNAPLs in groundwater in South Africa has now been concluded, with WRC support. The project yielded guidelines for a range of activities linked to the identification, characterisation and monitoring of DNAPL-contaminated sites.

This is important, as South Africa's aquifers are mainly fractured aquifers. Fracture or secondary aquifers refer to conditions where groundwater moves through a variety of joints, cracks, fractures and faults. The degree of fracturing of rocks in South Africa is a function of the tectonic history of the rocks, as well as the rock composition with a unique set of challenges in assessing the transport and destination of DNAPLs in the subsurface.

"Because of our complex aquifer systems, as well as the physical and chemical properties of these substances, accurate prediction of the behaviour of the chemicals is challenging," reports Dr Adams.

WRC-funded investigations into DNAPLs have found that the extent of the problem in South Africa's urban areas is potentially much greater than expected. The problem is not confined to large urban centres, as many of the potential sources (such as auto shops and dry cleaners) are found in all types of settlements across the country.

This means the development of groundwater resources to supply urban areas (one of the considerations under the Department of Water Affairs & Forestry's Water for Growth and Development Framework) is thus very likely to be affected by potential contamination from DNAPLs.

There are several groundwater contaminators which include agricultural chemicals (fertilisers, herbicides, pesticides); metallurgical processes; petrol service stations (underground torage tanks); manufacturing and processing and stormwater/sewer systems.

But DNAPLs are not just the concern of industry. WRC recently produced a layman's guide to DNAPLs in South Africa as even incorrectly disposing of used car oil can lead to groundwater pollution. A handbook aimed at site investigators, geohydrologists, site-owners and regulators also aims to give guidance on DNAPL-contaminated sites.

The next step is to look at light, non-aqueous liquids (LNAPLs) that will build on the completed DNAPL study.

Crucial partnerships were built with industry partners during the DNAPL study, and the petrochemical sector, for example, has made some of its sites available for research for the new LNAPL study.

It is hoped that the outcome of these investigations will do much to aid the sustainable management and prevention of pollution of South Africa's precious groundwater.

Lani van Vuuren is the Public Understanding of Science Officer at the Water Research Commission in South Africa